1. Field of the Invention
The present invention relates to an electroacoustic transducer of an electromagnetic type for use in a portable communication device, e.g., a cellular phone or a pager, for reproducing an alarm sound or melody sound responsive to a received call and for reproducing voices and the like, and a portable communication device including the electroacoustic transducer of an electromagnetic type.
2. Description of the Related Art
FIGS. 9A and 9B are plan and cross-sectional views showing a conventional electroacoustic transducer 2000 of an electromagnetic type (hereinafter referred to as an electromagnetic transducer).
The conventional electromagnetic transducer 2000 includes a cylindrical housing 107 and a disk-shaped yoke 106 disposed so as to cover the bottom face of the housing 107. A center pole 103, which forms an integral part of the yoke 106, is provided in a central portion of the yoke 106. A coil 104 is wound around the center pole 103. Spaced from the outer periphery of the coil 104 is provided an annular magnet 105, with an appropriate interspace maintained between the coil 104 and the inner periphery of the annular magnet 105 around the entire periphery of the coil 104. The outer peripheral surface of the magnet 105 is abutted to the inner peripheral surface of the housing 107. An upper end of the housing 107 supports a disk-shaped diaphragm 100 so that an appropriate interspace exists between the first diaphragm 100 and the magnet 105, the coil 104, and the center pole 103. A magnetic member 101 is provided on the diaphragm 100 so as to be concentric with the diaphragm 100.
Now, an operation of the above-described conventional electromagnetic transducer 2000 will be described.
In an initial state where no current flows through the coil 104, a magnetic path is formed by the magnet 105, the magnetic member 101, the center pole 103, and the yoke 106. As a result, the magnetic member 110 is attracted toward the magnet 105 and the center pole 103, up to a point of equilibrium with the elastic force of the diaphragm 100. If an alternating current flows through the coil 104 in this initial state, an alternating magnetic field is generated in the aforementioned magnetic path, so that a driving force is generated on the magnetic member 101. Such a driving force generated on the magnetic member 101 causes the magnetic member 101 to be displaced from its initial state, along with the fixed diaphragm 100, due to an interaction with an attraction force which is generated by the magnet 105 and the driving force. The vibration caused by such displacement transmits sound.
The lower limit of a frequency band to be reproduced by an electromagnetic transducer is generally dependent on the minimum resonance frequency of a vibrating system. A vibrating system as used herein refers to a group of elements included in an electromagnetic transducer which actually vibrate so as to produce sound. In the conventional electromagnetic transducer 2000, the minimum resonance frequency cannot be reduced to such a level that a low frequency signal, such as an audio signal, can be reproduced. The reason will be described below.
The minimum resonance frequency of the electromagnetic transducer 2000 is dependent on the stiffness of a vibrating system, which is obtained as a difference between an elastic force of the diaphragm 100 and an attraction force generated on the magnetic member 101 by the magnet 105.
FIG. 10 shows a relationship between the force-displacement characteristics curve of the diaphragm 100 and the attraction force generated on the magnetic member 101 by the magnet 105. In FIG. 10, the vertical axis represents a force while the horizontal axis represents a displacement of the diaphragm 100. An intersection A between a curve indicating the force-displacement characteristics of the diaphragm 100 and a curve indicating the attraction force generated on the magnetic member101 by the magnet105 represents a point where the elastic force of the diaphragm 100 is balanced with the attraction force. The minimum resonance frequency is dependent on a difference between the elastic force of the diaphragm 100 and the attraction force where the intersection A is regarded as an original point.
It is necessary to decrease the elastic constant of the diaphragm 100 in order to reduce the minimum resonance frequency. However, when the elastic constant of the diaphragm 100 is excessively small (i.e., no intersection A exists), the magnetic member 101 is trapped by the center pole 103 along with the diaphragm 100. Therefore, since the elastic constant must be the range in which the intersection A exists, the possible minimum resonance frequency is limited. Due to such a constraint, the minimum resonance frequency of the conventional electromagnetic transducer 2000 is typically about 2.5 kHz or more. Therefore, a low frequency signal, such as an audio signal, cannot be reproduced by the conventional electromagnetic transducer 2000.